What does "user interface" (UI) refer to?

The term "user interface" (UI) refers specifically to the means by which a user interacts with a system. This includes all the elements that facilitate user engagements, such as buttons, menus, text fields, icons, and more, which allow users to navigate and use the software or device effectively. Good UI design focuses on enhancing the usability and accessibility of systems, ensuring that users can complete tasks efficiently and comfortably.

The other options touch on aspects related to UI but do not capture its definition accurately. While the user's experience with hardware components might influence their interactions, it is not the definition of a user interface. Similarly, the software's graphical design is an important aspect of UI but does not encompass the entire concept, which includes all forms of interaction—not just visual elements. Lastly, the underlying code of an application refers to the programming and development side, which is crucial for functionality but is separate from how users interact with the system.